Power transformers are essential equipment for transmitting and distributing electrical power in power systems. They are used to step up or step down the voltage level, improve the power quality, and isolate different parts of the network. Power transformers are also expensive and complex devices that require proper installation, testing, and maintenance to ensure their optimal performance and safety.
This article will provide a comprehensive guide on how to install a power transformer, covering the following aspects:
- Standard and special tests for power transformers
- Site selection and preparation for power transformer installation
- Cleaning and drying of power transformer components
- Oil filling and filtering in the power transformer tank
- Fitting and fixing of power transformer accessories
- Earthing of power transformer
- Final inspection and commissioning of power transformer
The article will also provide some tips and best practices for installing power transformers, as well as some references to relevant standards and sources for further information.
What is a Power Transformer?
A power transformer is defined as a static electrical device that transfers electrical energy from one circuit to another by electromagnetic induction. It consists of two or more windings that are electrically insulated from each other and wound on a common magnetic core. The winding connected to the input voltage source is called the primary winding, while the winding connected to the output load is called the secondary winding.
A power transformer can have different types of core and winding arrangements, depending on the application and design requirements. Some common types of power transformers are:
- Core-type transformer: The windings are arranged concentrically around a laminated steel core that has two limbs.
- Shell-type transformer: The windings are arranged on two or more limbs of a laminated steel core that forms a closed magnetic circuit.
- Dry-type transformer: The windings and core are air-cooled and insulated by resin, varnish, or other materials that do not require liquid cooling.
- Liquid-filled transformer: The windings and core are immersed in a liquid medium, such as mineral oil or synthetic fluid, that provides cooling and insulation.
Power transformers can also be classified according to their voltage rating, winding configuration, cooling method, tap-changing mechanism, and other features.
Standard and Special Tests for Power Transformers
Before installing a power transformer, it is important to conduct various tests to verify its design, construction, and performance. These tests can be divided into two categories: standard tests and special tests.
Standard tests are performed for every power transformer unit to ensure its compliance with the specifications and standards. Standard tests include:
- Ratio test: This test measures the voltage ratio between the primary and secondary windings of the transformer at different tap positions.
- Polarity test: This test determines the relative direction of current flow in the primary and secondary windings of the transformer. It is important for parallel operation and connection of single-phase or three-phase transformers.
- Phase relation test: This test verifies the angular displacement between the primary and secondary voltages of a three-phase transformer. It is also essential for the parallel operation and connection of three-phase transformers.
- Excitation current test: This test measures the no-load current drawn by the primary winding of the transformer when the rated voltage is applied. It indicates the magnetizing characteristics and core losses of the transformer.
- Resistance test: This test measures the DC resistance of each winding of the transformer at a specified temperature. It is used to calculate the winding temperature rise, copper losses, and impedance values of the transformer.
- Impedance test: This test measures the impedance voltage or short-circuit voltage of the transformer by applying a reduced voltage across one winding and short-circuiting the other winding. It provides information for sizing breakers, fuses, relays, and other protective devices, as well as for coordinating protection schemes.
- Load loss test: This test measures the total power loss in the transformer under full-load conditions. It includes copper losses in the windings and stray losses in the core and other parts of the transformer.
- Regulation test: This test determines the voltage drop or rises in the secondary winding of the transformer when full-load current is applied at different power factors. It indicates the ability of the transformer to maintain constant output voltage under varying load conditions.
- Applied potential test: This test applies a high AC voltage between each winding and ground for a specified time to check the dielectric strength of the insulation system.
- Induced potential test: This test applies a high AC voltage between each pair of windings for a specified time to check the dielectric strength of the insulation system.
Special tests are performed for some power transformers depending on their application, design, or customer requirements. Special tests include:
- Impulse test: This test applies a high voltage impulse waveform, simulating lightning or switching surges, between each winding and ground or between each pair of windings. It evaluates the withstand capability of the transformer against transient overvoltages.
- Sound test: This test measures the sound level emitted by the transformer under no-load and load conditions. It is important for applications in residential and office areas and can also be used to detect any core problems.
- Temperature rise test: This test measures the temperature rise of the oil and windings of the transformer under rated load and ambient conditions. It helps to ensure that the design limits of the transformer are not exceeded and that the cooling system is functioning properly.
- Corona test: This test detects the presence and location of corona discharges in medium-voltage and high-voltage transformers. Corona is a phenomenon of partial discharge that occurs when the electric field exceeds a certain threshold, causing ionization of air and degradation of insulation.
- Insulation resistance test: This test measures the resistance of the insulation system of the transformer using a megohmmeter or an insulation tester. It indicates the dryness and cleanliness of the insulation and can be used as a benchmark for comparison with future readings.
- Dissolved gas analysis: This test analyzes the gases dissolved in the transformer oil, such as hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, carbon dioxide, nitrogen, and oxygen. It helps to diagnose the condition and faults of the transformer, such as overheating, arcing, partial discharge, and oil degradation.
Site Selection and Preparation for Power Transformer Installation
The site selection and preparation for power transformer installation are crucial steps that affect the performance, reliability, and safety of the transformer. The following factors should be considered when choosing and preparing a site for power transformer installation:
- Accessibility: The site should be easily accessible by road or rail for transportation and delivery of the transformer and its components. The site should also provide sufficient space for unloading, handling, lifting, and moving the transformer and its accessories.
- Foundation: The site should have a strong and level foundation that can support the heavy weight of the transformer and its oil. The foundation should also prevent water accumulation under the transformer and provide drainage facilities for oil spills or leaks.
- Clearance: The site should provide adequate clearance from the earth and other live parts for all live parts of the transformer, such as bushings, terminals, tap changers, etc. The clearance should comply with the relevant standards and regulations for safety and insulation coordination.
- Ventilation: The site should provide sufficient ventilation for natural or forced cooling of the transformer. If the transformer is installed indoors, the room should have adequate air inlets and outlets to ensure proper air circulation. If the transformer is installed outdoors, it should be protected from direct sunlight, rain, snow, dust, etc.
- Noise: The site should minimize noise pollution from the transformer to nearby residential or office areas. The noise level of the transformer can be reduced by using sound barriers, enclosures, or silencers.
- Fire protection: The site should have fire protection measures in place to prevent or control fire hazards from the transformer. These measures may include fire extinguishers, sprinklers, fire alarms, firewalls, etc.
Cleaning and Drying of Power Transformer Components
Before installing a power transformer, it is essential to clean and dry all its components to remove any dust, dirt, moisture, or foreign materials that may affect its insulation properties or performance. The following procedures should be followed for cleaning and drying power transformer components:
- Main tank: The main tank of the transformer should be cleaned from inside and outside using a clean cotton cloth or compressed air. If any rust is found on the interior surface of the tank, it should be removed by sandblasting or scraping. The main tank should be filled with dry and warm oil under vacuum until its temperature is 5°C to 10°C above ambient. This should be done before allowing external access to the interior of the tank. The warm and dry oil should be circulated in the tank all the time the transformer remains open to the atmosphere. The oil pump and all joints in the oil pipework should be airtight to prevent air ingress into the transformer.
- Core and windings: The core and windings of the transformer should be cleaned using a clean cotton cloth or compressed air. Any loose particles or fibers should be removed carefully. The core and windings should be dried using a streamlined filter machine that has a vacuum chamber and an oil heater. The filter machine should be filled with fresh transformer oil supplied by the manufacturer and circulated inside the machine several times until it meets the required specifications. The filter machine should then be connected to the main tank, and the oil should be circulated through the transformer and filter machine several times until the insulation resistance value of the oil reaches a satisfactory level. The vacuum pump of the filter machine should create a high vacuum, preferably above 710 mm of mercury, in the main tank. An additional vacuum pump can be connected at the top cover of the tank to enhance the drying process. The oil temperature in the filter machine should not exceed 85°C, and the oil temperature in the tank should not exceed 75°C to avoid oil decomposition. The drying process should continue until all moisture is removed from the core, windings, and oil.
- Bushings: The bushings of the transformer should be checked for any damage or crack before installation. They should be cleaned using a dry cotton cloth or compressed air. The bushings should be clamped to the tank with even pressure on the flange and gasket by tightening the nuts diagonally. The power connection at the stud of the bushing should be flexible enough to absorb any vibration. If arcing horns are supplied with the bushing, they should be fitted and adjusted according to the manufacturer’s instructions.
- Conservator tank: The conservator tank of the transformer, where fitted, should be assembled with its pipework, making sure that all joints are oil tight and clean. The mechanism of the float-type oil gauge inside the conservator should be unlocked by turning the locking bolt in the direction indicated on the plate. The conservator tank should be filled with transformer oil up to the level indicated by the oil gauge on the conservator at a specified temperature.
- Radiators: The radiators of the transformer should be washed with clean dry oil before fitting to the main body of the transformer. They should be fitted with proper gaskets and bolts, ensuring that no air is trapped inside the radiators. The air should be released by loosening the air release pockets at the top of the header and each radiator unit. The radiators should not be subjected to a vacuum during the drying process of the main tank. The interconnecting valves between the tank and the radiator units should be opened after the drying process is completed. The cooling fans and oil-circulating pumps, if provided, should also be fitted and wired according to the cooler control drawings.
Installing a power transformer is a complex and critical task that requires careful planning, preparation, and execution. This article has provided a comprehensive guide on how to install a power transformer, covering the standard and special tests, site selection and preparation, cleaning and drying of components, oil filling and filtering, fitting and fixing of accessories, earthing, final inspection, and commissioning of the transformer.
By following this guide, one can ensure that the power transformer is installed properly and safely and that it performs efficiently and reliably in the power system.